Glass substrate, magnetic recording medium and method of manufacturing the magnetic recording medium

ABSTRACT

In a glass substrate for use in a magnetic recording medium, the glass substrate has a pair of principal surfaces opposite to each other and a peripheral side area contiguous to the principal surfaces. The peripheral side area has a side end wall and intermediate regions between the side end wall and the principal surfaces. In this case, at least one of the intermediate regions and the side end wall is formed by a mirror finished surface which has a surface roughness Ra not greater than 1 μm or a surface roughness Rmax not greater than 4 μm. With this structure, no particles are generated from the peripheral side area because the side end surface of the glass substrate is mirror finished. Consequently, reproduction is performed without reproduction errors because of no generation of a thermal asperity and a head crash can be sufficiently avoided because of no projections which might result from the particles.

This is a divisional of Application Ser. No. 08/941,296 filed Sep. 30,1997 now U.S. Pat. No. 6,096,405, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a magnetic recording medium used for acomputer and the like.

Heretofore, an aluminum substrate has been widely used for a magneticrecording medium, such as a magnetic disk. However, the aluminumsubstrate has been gradually replaced by a glass substrate which isadvantageous in strength and flatness as compared with the aluminumsubstrate.

On the other hand, a thin-film head has been widely used as a magnetichead. However, the thin-film head tends to be recently replaced by amagneto-resistive head (MR head) and a giant magneto-resistive head (GMRhead). Therefore, development would be directed to a magnetic recordingmedium which can be matched with the magneto-resistive head. Such amagnetic recording medium often has a glass substrate.

However, reproduction errors very often occur and a reproduction becomesmore difficult as a flying height becomes lower when themagneto-resistive head runs along the magnetic recording medium of theglass substrate.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a magnetic recording mediumwhich is capable of reducing a reproduction error even if a flyingheight of a magnetic head becomes low.

As mentioned above, the reproduction error often occurs when the flyingheight of the magnetic head becomes low. Therefore, close investigationhas been carried out about the cause of the reproduction error.According to the inventors' experimental studies, it is found out thatparticles are attached on a principal surface of a glass substrate andprojections are formed due to the particles on the magnetic recordingmedium. Such projections bring about a thermal asperity which causes themagneto-resistive head to generate heat. As a result, a resistivity ofthe head is fluctuated and thereby, an adverse influence is given toelectro-magnetic conversion.

Further, the thermal asperity is often generated for the magnetic diskafter a glide test for checking a head crash. In this case, variousparticles bring about the thermal asperity. For example, such particlesmay include a particle which gives an adverse influence when the head isafloat. Such a particle comprises an angular or irregular configuration.

Subsequently, the cause that the particles are attached to theconventional glass substrate has been investigated. As a result of theinvestigation, it is found out that the particles generated from a sideend surface of the glass substrate are attached to the principal surfaceof the glass substrate. This is because the end side surface is rough ascompared with a principal surface of the glass substrate. Specifically,when the glass substrate is got in and out of a housing of a materialsuch as a polycarbonate, the side end surface of the glass substrate isinevitably contacted with an internal surface of the housing. Suchcontacts of the side end surface with the housing give rise togeneration of the particles and the generated particles are attached onthe principal surface of the glass substrate.

Conventionally, an attention has been generally paid for the side endsurface only from the viewpoint of a mechanical strength of the glasssubstrate. In contrast, an attention is paid in U.S. Pat. No. 5,569,518for avoiding generation of the particles by chemically etching orpolishing the side end surface into a chemical polished surface. It isknown in the art that such a chemical polished surface is rough andexhibits a mat (or dull) finished surface. As a result, it has beenconfirmed that the side end surface must be precisely polished bymechanical polishing to avoid generation of the particles.Conventionally, the side end surface is processed by chemical etching.However, the chemical etching process is not enough to suppressgeneration of the particles.

Herein, it is assumed that a magnetic recording medium which ismanufactured by the use of the glass substrate is shaped in a disk whichhas an inner periphery and an outer periphery. In this event, the innerand the outer peripheries are irregularly etched, which might result indeviation of an axis.

On the other hand, the present invention points out that generation ofparticles can be prevented by mechanically polishing the side surfaceinto a mirror finished surface without any deviation of an axis in amagnetic recording medium.

Specifically, according to this invention, the glass substrate has apair of principal surfaces opposite to each other and a peripheral sidearea contiguous to the principal surfaces. The peripheral side area hasa side end wall and intermediate regions between the side end wall andthe principal surfaces. In this case, at least one of the intermediateregions and the side end wall is formed by a mirror finished surfacewhich has a surface roughness Ra not greater than 1 μm or a surfaceroughness Rmax not greater 4 μm.

In this invention, no particles are generated because the side endsurface of the glass substrate is mirror finished. Consequently,reproducing function is not reduced by the thermal asperity. Further, ahead crash can be sufficiently prevented since no projections are formeddue the particles on the magnetic recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a part of a glass substrate; and

FIG. 2 shows a graphical representation for use in illustrating resultsof glide yield losses of various samples to specify the samplesaccording to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, description will be made about a glass substrate 1of this invention.

The glass substrate 1 has an end portion in section, as illustrated inFIG. 1. In FIG. 1, the glass substrate 1 has principal surfaces 3opposite to each other and a peripheral side area 2 contiguous to theprincipal surfaces 3. The peripheral side area 2 has a side end wall Aand intermediate regions B and C between the side end wall A and each ofthe principal surfaces 3.

In the illustrated example, each of intermediate regions B and C isintentionally chamfered into a chamfered portion by a chamferingprocess, as will later be discussed. However, the intermediate regions Band C may be unintentionally chamfered during a grinding or a polishingprocess, as will later be discussed also. Herein, it is to be noted thatthe side end wall A and the intermediate regions B and C are all mirrorfinished by mechanical polishing and therefore have mechanical polishedsurfaces specified by a surface roughness Ra which is not greater than 1μm, where Ra is representative of the center-line mean roughness(defined in Japanese Industrial Standard JIS B 0601). The mechanicalpolished surfaces will also be called mirror finished surfaceshereinunder because they are subjected to a mirror finished process.However, such a mirror finished surface may be formed on at least one ofthe side end wall A and the intermediate regions B and C. In this event,the surface roughness Ra falls preferably in the range of 0.001 to 0.8μm, and more preferably, in the range of 0.001 to 0.5 μm.

Alternatively, the mirror finished surface may be represented by asurface roughness Rmax, where Rmax is defined as a maximum heightrepresentative of a difference between a highest point and a lowestpoint, instead of Ra. In this event, the surface roughness Rmax may benot greater than 4 μm. Preferably, the maximum height Rmax falls withinthe range between 0.01 and 2 μm, and more preferably, within the rangebetween 0.01 and 1 μm.

Thus, the glass substrate 1 according to this invention has the mirrorfinished surfaces each of which is specified by the surface roughness Ranot greater than 1 μm or the surface roughness Rmax not greater than 4μm. Under the circumstances, it has been confirmed that no projectionsare generated on the principal surfaces 3, resulting from the surfaceroughness of the side end wall A and the intermediate regions B and C ofthe glass substrate 1. Consequently, it has been found out that nothermal asperity is generated.

Further, rounded surfaces 4 are formed between each of the principalsurfaces 3 and each of the intermediate regions B and C. In addition,rounded surfaces 4 are formed between each of the intermediate regions Band C and the side end wall A. Each rounded surface 4 has a radiusbetween 0.2 and 10 mm, preferably between 0.2 and 2 mm, and morepreferably between 0.2 and 1 mm. Generation of the particles cancompletely be prevented by forming the rounded surfaces 4. Specifically,if angular surfaces 5 are formed between each of the principal surfaces3 and each of the intermediate regions B and C or between each of theintermediate regions B and C and the side end wall A, the angularsurfaces 5 might generate the particles when they are contacted with thehousing. Therefore, according to this invention, the rounded surfaces 4are formed by mechanically polishing the angular surfaces 5 to avoidgeneration of the particles. The rounded surface 4 has preferably thesurface roughness Rmax of 0.01-2 μm and the surface roughness Ra of0.001-1 μm, more preferably, the surface roughness Rmax of 0.01-1 μm andthe surface roughness Ra of 0.001-0.8 μm, and still more preferably, thesurface roughness Rmax of 0.01-1 μm and the surface roughness Ra of0.001-0.5 μm.

A mirror finish process is performed for the peripheral side area 2 ofthe glass substrate 1 to obtain the mirror finished surface as mentionedabove. Specifically, the mirror finish process is carried out bymechanical polishing, such as brushing, buffing, polishing by the use ofa polishing pad, such as a soft polisher and a hard polisher, and asolid or a liquid polishing substance, such as loose abrasives, bondedabrasives, slurry, and the like. At any rate, the mechanical polishingmay be performed by combining mechanical polishing of different typesand mechanical polishing carried out by the use of different polishersin grain size and types to obtain the mirror finished surface having theabove-mentioned surface roughness. The mechanical polishing may bereferred to as physical polishing or dynamical polishing.

For example, the soft polisher may include a suede, a velour, or thelike, while the hard polisher may include hard velour, foamed urethane,pitch impregnated suede, or the like. Further, the abrasives mayinclude, for example, cerium oxide (CeO₂), alumina (γ—Al₂O₃), red oxide(Fe₂O₃), chromium oxide (Cr₂O₃), zirconium oxide (ZrO₂), and titaniumoxide (TiO₂).

In the illustrated example, the intermediate regions (chamferedportions) B and C are formed on both sides of the side end wall A.Thereafter, the mirror finish process is performed for the intermediateregions B and C. As a result, no particles are generated from the sideend wall A and the intermediate regions B and C even if the glasssubstrate 1 contacts with a housing at the peripheral side area 2 whenthe glass substrate 1 is got in and out of the housing.

The mirror finish process due to the mechanical polishing for theperipheral side area 2 may be carried out after or before the principalsurfaces 3 of the glass substrate 1 are lapped or polished. In general,the polishing or lapping process is divided into (1) a rough grindingstep, (2) a lapping step, (3) a first polishing step, (4) a secondpolishing step. Taking the above into consideration, the above-mentionedmirror finish process may be performed after each of the steps (1), (2),(3), and (4). For example, the mirror finish process may be performedafter the rough grinding step. Alternatively, the mirror finish processmay be performed after the lapping step, the first polishing step, orthe second polishing step. Where the mirror finish process is performedbefore the lapping step, the peripheral side area 2 is often slightlyrough by sands used in the lapping step.

In addition, it is preferable that the principal surface 3 of the glasssubstrate 1 has the surface roughness Ra not greater than 2 nm toprevent the thermal asperity.

No limitation is imposed as to a kind, a size and a thickness of theglass substrate 1 in this invention. The glass substrate 1 may be formedby glass or glass ceramics. In this case, the glass may be, for example,aluminosilicate glass, soda-lime glass, soda aluminosilicate glass,aluminoborosilicate glass, borosilicate glass, silicate glass and chainsilicate glass, while the glass ceramics may be, for example,crystallized glass. By way of example, the aluminosilicate glass isexemplified and may contain, by weight, 62-75% of SiO₂, 5-15% of Al₂O₃,4-10% of Li₂O, 4-12% of Na₂O, 5-15% of ZrO₂ as main components. Inaddition, the glass includes a weight ratio of Na₂O/ZrO₂ which fallswithin a range between 0.5 and 2.0 and a weight ratio of Al₂O₃/ZrO₂which falls within a range between 0.4 and 2.5 so as to chemicallyreinforce the glass substrate. Further, such chemically reinforced glasspreferably contains, by mol %, 57-74% of SiO₂, 0-2.8% of ZnO₂, 3-15% ofAl₂O₃, 7-16% of LiO₂, and 4-14% of Na₂O so as to remove projections onthe principal surfaces 3 which results from insoluble substances ofZrO₂. The chemically reinforced aluminosilicate glass which has theabove-mentioned composition is excellent in transverse bending strengthand Knoop hardness. In addition, such a glass has a compressive stresslayer deep enough.

In this embodiment, the chemical reinforcement can be made by performingan ion exchange method at a low temperature for the glass substrate 1 toimprove properties against impact and vibration. Instead, the knownchemical reinforcement can be used to chemically reinforce the glasssubstrate 1. However, the ion exchange method is preferable as comparedwith the latter method from the viewpoint of a glass transition pointbecause the former is carried out at a temperature less than the glasstransition point. A fused salt used for the chemical reinforcementincludes a potassium nitrate, a sodium nitrate, a nitrate mixed withthem. The glass substrate 1 according to this invention is also used notonly for a magnetic recording medium but also a photo-magnetic disk andan electro-optical disk, such as a photo disk. Especially, the glasssubstrate is effective to the photo-magnetic disk which is seriouslyinfluenced by particles attached thereto.

Subsequently, description will be made about a magnetic recording mediummentioned above.

Herein, such a magnetic recording medium has at least one magnetic layerformed on the glass substrate 1 as well known in the art. In thisinvention, no particles are generated from the side end wall A and theintermediate regions B and C because the side end wall A and theintermediate regions B and C are subjected to the mirror finish process,as mentioned above. This shows that no projections are left on themagnetic layer due to generation of the particles, even when themagnetic layer is formed on the glass substrate 1. Consequently, thehead crash can be sufficiently prevented when the reproduction isperformed by the magneto-resistive (MR) head. Further, no reproductionerror occurs because no defect due to the particle remains on themagnetic layer.

Specifically, the magnetic recording medium is formed by successivelydepositing an underlying layer, a magnetic layer, a protection layer anda lubricating layer on the glass substrate 1 which has the desiredflatness and surface roughness as mentioned above and which is subjectedto chemical reinforcement.

The underlying layer is selected in relation to the magnetic layer. Atleast one metal selected from a group consisting of nonmagnetic metals,Cr, Mo, Ta, Ti, W, V, B, Al may be used as a material of the underlyinglayer. The metal Cr or the Cr alloy is preferably used as the materialof the underlying layer to enhance a magnetic characteristic where themagnetic layer includes Co as a main component. In addition, theunderlying layer may not always be formed by a single layer but may beformed by a multi-layer composed of a plurality of identical ordifferent layers. For example, deposition may be made as the underlyinglayer formed by the multi-layer such as Cr/Cr, Cr/CrMo, Cr/CrV, CrV/CrV,Al/Cr/CrMo, Al/Cr/Cr, Al/Cr/CrV, and Al/CrV/CrV.

In this invention, no limitations are imposed as to the magnetic layeralso.

The magnetic layer may be, for example, a layer which contains Co as amain component and which has a composition selected from CoPt, CoCr,CoNi, CoNiCr, CoCrTa, CoPtCr, CoNiPt, CoNiCrPt, CoNiCrTa, CoCrTaPt, andCoCrPtSiO. In addition, the magnetic layer has a multi-layer structure(for example, CoPtCr/CrMo/CoPtCr, CoCrTaPt/CrMo/CoCrTaPt). Such astructure is obtained by dividing a magnetic film by a nonmagnetic film(for example, Cr, CrMo, CrV) to reduce a noise, as known in the art. Themagnetic layer for the magneto-resistive head (MR head) or the giantmagneto-resistive head (GMR head) contains impurity elements selectedfrom a group consisting of Y, Si, rare-earth elements, Hf, Ge, Sn andZn, oxides of these impurity elements in addition to the Co-based alloy.

Further, the magnetic layer may have a granular structure whereinmagnetic grains, such as Fe, Co, FeCo and CoNiPt, are dispersed in thenonmagnetic film comprising a ferrite-based material, an iron-rareearth-based material, SiO₂, and BN. Further, the magnetic layer may havea recording form of either an in-plane magnetization type or aperpendicular magnetization type.

No restrictions are imposed as to the protection layer also according tothis invention. Specifically, the protection layer may be formed by achromium film, a chromium alloy film, a carbon film, a zirconia film anda silica film and may be successively deposited on the glass substrate 1together with the underlying layer and the magnetic layer by the use ofthe known in-line sputtering apparatus. The protection layer may beformed by a single layer or a multi-layer including a plurality oflayers of an identical material or different materials.

In addition, the other protection layer, such as a SiO₂ film, may beused instead of the above protection layer. Such a SiO₂ film may beformed on the chromium film by dispersing colloidal silica fine grainsin tetraalkoxysilane diluted with an alcohol-based solvent andthereafter by coating and baking the dispersed grains.

Moreover, the lubricating layer is not restricted to the above. Forexample, the lubricating layer is formed by diluting perfluoropolyether(PFPE) with a solvent, such as freon-based solvent, and applying it onthe medium surface by a dipping method, a spin coating method, or aspraying method, and firing the medium.

FIRST EXAMPLE

(1) Roughing Step by Grinding and Polishing

First, a sheet glass of aluminosilicate is formed by a down draw methodand is cut by a grinding wheel into a disc-shaped glass substrate whichhas a diameter of 96 mm and a thickness of 3 mm. The glass substrate islapped or polished to a diameter of 96 mm and a thickness of 1.5 mm by arelatively rough diamond grindstone. In this case, the disc-shaped glasssubstrate may be formed by directly pressing a melting glass with acope, a drag and a drum instead of the down draw method.

In this event, the above-mentioned glass which is chemically reinforcedcontains, by mol %, 57-74% of SiO₂, 0-2.8% of ZnO₂, 3-15% of Al₂O₃7-16%of LiO₂, and 4-14% of NaO₂ as main components. Further, the chemicallyreinforced glass contains, by weight, 63% of SiO₂, 14% of Al₂O₃, 6% ofLi₂O, and 10% of Na₂O as main components. The above-mentionedcomposition can be expressed in terms of a mol representation andcomprises, by mol %, 67% of SiO₂, 1.0% of ZnO₂, 9.0% of Al₂O₃, 12.0% ofLiO₂ and 10.0% of NaO₂.

Subsequently, the both principal surfaces 3 of the glass substrate 1 areground or lapped by a diamond grindstone having grains smaller thanthose of the above grindstone. In this case, a load was set to theextent of 100 Kg. Thereby, the principal surface 3 of the glasssubstrate 1 was ground into a surface roughness Rmax (JIS B 0601) ofabout 10 μm.

Next, an opening was formed at a center portion of the glass substrate 1by using the cylindrical grindstone. Further, the outer side end surfaceis ground to a diameter of 95 mm. Thereafter, the outer and inner sideend surfaces were chamfered. In this case, the side end wall A of theglass substrate 1 had a surface roughness Rmax of about 14 μm.

(2) Mirror Finishing Step of the Side End Surface

The internal and outer side walls of the glass substrate 1 were polishedby the use of a brush and loose abrasives by rotating the glasssubstrate 1 so that the both side end walls had the surface roughnessRmax of about 1 μm and the surface roughness Ra of about 0.3 μm. Next,the glass substrate 1 was washed with water after the mirror finishprocess of the side end walls.

(3) Lapping Step

The lapping step was performed for the glass substrate 1 to improvedimension and shape accuracy. The lapping step is carried out by using alapping apparatus. In this case, the lapping step is conducted two timesby changing grain degrees from #400 to #1000. Specifically, the lappingwas performed for the both principal surfaces 3 of the glass substrate 1so that the principal surfaces 3 had a surface accuracy of 0-1 μm andthe surface roughness Rmax of about 6 μm. In this event, the lapping wascarried out by rotating an inner gear and an outer gear by the use ofalumina grains having a grain degree of #400 on the condition that theload L was kept at about 100 Kg. Next, the lapping is performed bychanging the grain degree of the alumina grain into #1000. As a result,the surface roughness Rmax becomes about 2 μm. Subsequently, the glasssubstrate 1 was immersed in washing units by using natural detergent andwater to be washed.

(4) First Polishing Step

Next, first polishing was performed by a polishing apparatus to remove adefect and a distortion remaining in the above lapping process.Specifically, a hard polisher (which may be a cerium impregnated foamedurethane pad, such as MHC15 made by Speedfam) was used as a polisher andthe first polishing was performed the following polishing condition.

Polishing liquid: oxide cerium+water

Load: 300 g/cm² (L=238 kg)

Polishing time: 15 minutes

Removing amount: 30 μm

Revolution of lower surface plate: 40 rpm

Revolution of upper surface plate: 25 rpm

Revolution of inner gear: 14 rpm

Revolution of outer gear: 29 rpm.

The glass substrate 1 was washed by being successively dipped in washingunits of natural detergent, pure water, IPA (isopropyl alcohol), IPA(vapor drying) after the first polishing.

(5) Second Polishing Step

Next, second polishing was conducted by changing the above hard polisherinto a soft polisher (which may be a polishing pad of a suede type, suchas Polylax made by Speedfam) by using the polishing apparatus used inthe first polishing process. The polishing condition is similar to thefirst polishing step except for the load of 100 g/cm², the polishingtime of 5 minutes and the removing amount of 5 μm. The glass substrate 1was immersed in washing units of the natural detergent, the pure water,the IPA (isopropyl alcohol), the IPA (vapor drying) to be washed thereinafter the second polishing step. In this case, a supersonic wave wasapplied to each of the washing units.

(6) Chemical Reinforcing Step

Next, a chemical reinforcing step was performed for the glass substrate1 after the polishing step was completed. First, a chemical reinforcingsolution was prepared by mixing potassium nitrate (60%) with sodiumnitrate (40%). The chemical reinforcing solution was heated up to 400°C. The glass substrate 1 which was washed and preheated to 300° C. wasdipped in the chemical reinforcing solution for 3 hours. The chemicalreinforcing step was carried out so that the entire surface of the glasssubstrate 1 was chemically reinforced with a plurality of glasssubstrates retained at the end surface in a holder. Under thecircumstances, lithium ions and sodium ions on a surface layer of theglass substrate 1 were replaced by sodium ions and potassium ions in thechemical reinforcing solution by dipping each glass substrate 1 in thethe chemical reinforcing solution. Thus, the glass substrate 1 ischemically reinforced. A compressive stress layer formed in the surfacelayer of the glass substrate 1 had a thickness of about 100-200 μm.Next, the chemically reinforced glass substrate 1 was dipped in a watertank of 20° C., quickly cooled and retained for 10 minutes.Subsequently, the cooled glass substrate 1 was dipped in a sulfuric acidheated up to 40° C., and was washed by the supersonic wave. The surfaceroughness (Ra, Rmax) of the peripheral side area 2 of the glasssubstrate 1 at the intermediate region (chamfered portion) B (length:0.15 mm), the intermediate region (chamfered portion) C (length: 0.15mm) and the side end wall A (length: 0.35 mm) are shown in Table 1. InTable 1, the surface roughness Ra of the intermediate region B is 0.2μm, while the surface roughness Rmax of the intermediate region B is 1.9μm. Further, the surface roughness Ra of the intermediate region C is0.15 μm, while the surface roughness Rmax of the intermediate region Cis 2.9 μm. In addition, the surface roughness Ra of the side end wall Ais 0.35 μm, while the surface roughness Rmax of the side end wall A is3.4 μm. Also, the principal surface 3 of the glass substrate 1 has thesurface roughness Ra of 0.5-1 nm. In addition, the surface of the glasssubstrate 1 is precisely tested. As a result of the test, no particleswhich are the cause of the thermal asperity exist on the principalsurface 3.

TABLE 1 Ra (μm) Rmax (μm) Side End Wall A 0.35 3.4 Intermediate Region B0.2 1.9 Intermediate Region C 0.15 2.9

Measuring Length: 0.8 mm

In addition, it is to be noted that the side end wall A is 0.45 mm thickwhen the glass substrate has a diameter of 3.5 inches. In this case,similar result has been confirmed.

(7) Magnetic Disc Manufacturing Step

A texture layer, a CrN underlying layer, a CrMo underlying layer, aCoPtCrTa magnetic layer and a C protection layer were successivelydeposited on the both principal surfaces 3 of the glass substrate 1 byusing the known in-line sputtering apparatus to obtain a magnetic disk.Next, a glide test was performed in connection with the obtainedmagnetic disk. As a result of the test, no hit and crash occur. In thiscase, the hit means that the magnetic head touches projections on thesurface of the magnetic disk, and the crash means that the magnetic headcollides with the projections on the surface. Further, it is confirmedthat no defects due to the particles are generated on the magneticlayer.

Also, the glide test was performed for the first example wherein theperipheral side area 2 of the glass substrate 1 was mirror finished, afirst comparative example wherein the peripheral side area end 2 wasetched, and a second comparative example wherein neither polishing noretching was performed. The test result is shown in Table 2 as a resultof the glide test. Concretely, the yield of the first example was 95% ormore. In contrast, the yield of each of the first and second comparativeexamples was 85%. From Table 2, it is understood the first example issuperior to the first and the second examples. Thus, the conventionalmethod of etching the peripheral side area of the glass substrate cannot accomplish the object of this invention.

TABLE 2 1st 2nd Comparative Comparative 1st Example Example ExampleYield of 90% or more 87% or less 87% or less Glide Test

In addition, a reproduction test was performed for the magnetic diskafter the glide test. As a result of the test, it has been confirmedthat no reproduction errors due to the thermal asperity take place inconnection with the samples (500 samples) according to this invention.

SECOND AND THIRD EXAMPLES

Second and third examples are similar to the first example except thatsoda-lime glass (the second example) and soda aluminosilicate glass (thethird example) are replaced by the aluminosilicate glass of the firstexample. As a result, the outer and inner side end surfaces of the glasssubstrate 1 have the surface roughness Rmax of 1.5 μm in case ofsoda-lime glass (the second example). Although the soda-lime glasssubstrate had a slightly rough surface as compared with thealuminosilicate glass, no practical problem accrued in such a glasssubstrate.

FOURTH EXAMPLE

An underlying layer of Al (50 Å)/Cr (1000 Å)/CrMo (100 Å), a magneticlayer of CoPtCr (120 Å)/CrMo (50 Å)/CoPtCr (120 Å) and a Cr (50 Å)protection layer are successively deposited on the both principalsurfaces 3 of the glass substrate 1 obtained in the first example by theknown in-line sputtering apparatus. The above substrate is dipped in anorganosilicon compound solution (mixed liquid with water, IPA andtetraethoxy silane) wherein silica fine grains (grain diameter of 100 Å)are dispersed. Next, the protection layer of SiO₂ textured was formed bybaking. Further, the protection layer was subjected to a dipping processwithin a lubricating agent of perfluoropolyether to obtain the magneticdisk for the MR head.

The glide test was carried out for the obtained magnetic disk. As aresult of the test, neither hit nor crash occur. Further, it has beenconfirmed that no defect was formed on the magnetic layer. In addition,no reproduction errors due to the thermal asperity were generated as aresult of reproduction test.

FIFTH EXAMPLE

A fifth example is similar to the fourth example except that Al/Cr/Crwas used as the underlying layer and CoNiCrTa is used as the magneticlayer. It has been confirmed that a magnetic disk like in the fourthexample was obtained in the fifth example also.

Referring to FIG. 2, the glide test has been performed in connectionwith first through twelfth samples 1 to 12 with flying heights keptwithin a range between 1.1 and 1.3 (microinches). Herein, one hundredglass substrates have been prepared as each of the first through thetwelfth samples 1 to 12. It is to be noted that the first, the second,the fifth, the eighth, the ninth, the tenth, the eleventh, and thetwelfth samples 1, 2, 5, 8, 9, 10, 11, and 12 had the surfaceroughnesses Ra and Rmax defined by this invention while the third, thefourth, the sixth, and the seventh samples 3, 4, 6, and 7 had thesurface roughnesses Ra and Rmax falling outside this invention, wheneach of the surface roughnesses Ra and Rmax is represented by an averagevalue of the one hundred glass substrates. More specifically, the first,the second, the fifth, the eighth, the ninth, the tenth, the eleventh,and the twelfth samples 1, 2, 5, 8, 10, 11, and 12 had the surfaceroughnesses Ra and Rmax which are equal to 0.03 and 0.44; 0.18 and 1.80;0.06 and 3.25; 0.53 and 2.50; 0.71 and 3.56; 0.09 and 0.98; 0.06 and0.85; and 0.05 and 0.50 μm, respectively. On the other hand, the third,the fourth, the sixth, and the seventh samples had the surfaceroughnesses Ra and Rmax which are equal to 1.88 and 10.05; 1.53 and7.45; 1.72 and 8.23; and 2.01 and 11.75 μm, respectively, and matfinished surfaces.

Under the circumstances, glide yield losses were measured during theglide test mentioned above by counting defects on each of the firstthrough the twelfth samples 1 to 12 and were represented by (%), asshown by histograms in FIG. 2. The first, the second, the fifth, theeighth, the ninth, the tenth, the eleventh, and the twelfth samples 1,2, 5, 8, 10, 11, and 12 according to this invention exhibited the glideyield losses of 1.625, 5.75, 8.5, 7.895, 9.625, 3.5, 3.25, and 1.825%,respectively, while the third, the fourth, the sixth, and the seventhsamples 3, 4, 6, and 7 exhibited the glide yield losses of 14.625,13.875, 14.375, and 15.125%, respectively. At any rate, the samplesaccording to this invention exhibited the glide yield losses smallerthan 10%.

In order to measure defects on a magnetic recording medium, varioustests have been usually carried out by the use of a magneto-resistive(MR) head kept at a flying height of 50 nm and a rotation speed of 7m/sec and may be collectively called a certification test. Actually, thecertification test includes a positive modulation test, a negativemodulation test, a missing test, an extra test, and a spike test, all ofwhich are known in the art. Such tests also serve to evaluate thethermal asperity of each magnetic recording medium. Herein, each of themissing test, the spike test, and the extra test is helpful to classifythe defects into a correctable defect, an uncorrectable defect, and along defect.

Prior to each test, provision has been made about twenty-five of themagnetic recording media which have been polished in the above-mentionedmanner, together with twenty-five unpolished magnetic recording media.Results of the above-mentioned tests have been averaged and summed upabout the polished and the unpolished magnetic recording media. Underthe circumstances, it has been found out that an average sum value ofdefects on each surface of the polished magnetic recording media hasbeen equal in number to 9.56 while the average sum value of defects oneach surface of the unpolished magnetic recording media has been equalin number to 16.4. In comparison with both the average sum values, ithas been readily understood that a reduction rate of the defects in thepolished magnetic recording media has been equal to 41.70%. This showsthat the polished magnetic recording media according to this inventionare remarkably improved in thermal asperity, as compared with theunpolished magnetic recording media.

While this invention has thus far been described in conjunction withseveral examples thereof, it will readily be possible for those skilledin the art to put this invention into practice in various other manners.For example, the mechanical polished surface, namely, the mirrorfinished surface may be formed on at least one of the inner and theouter peripheral surfaces of the magnetic recording medium.

What is claimed is:
 1. A disk-shaped glass substrate for use in amagnetic recording medium, said glass substrate having a pair ofprinciple surfaces opposite to each other and an internal peripheralside area and an outer peripheral side area contiguous to said principlesurfaces, each of the peripheral side area having a side end wall andintermediate regions between the side end wall and the principlesurfaces, said principle surfaces having smooth surfaces which aresuitable for use in a magnetic recording medium wherein, at least one ofsaid intermediate regions and said side end wall of at least the outerperipheral side area is formed by a mirror finished surface which have asurface roughness Ra not greater than 1 μm in order to substantiallyreduce particles generated from the peripheral side area, where Ra isrepresentative of the center-line mean roughness.
 2. A disk-shaped glasssubstrate for use in a magnetic recording medium, said glass substratehaving a pair of principle surfaces opposite to each other and aninternal peripheral side area and an outer peripheral side areacontiguous to said principle surfaces, each of the peripheral side areahaving a side end wall and intermediate regions between the side endwall and the principle surfaces, said principle surfaces having smoothsurfaces which are suitable for use in a magnetic recording medium,wherein, at least the intermediate regions of at least the outerperipheral side area is formed by a mirror finished surface which have asurface roughness Ra not greater than 1 μm in order to substantiallyreduce particles generated from the peripheral side area, where Ra isrepresentative of the center-line mean roughness.
 3. A disk-shaped glasssubstrate for use in a magnetic recording medium, said glass substratehaving a pair of principle surfaces opposite to each other and aninternal peripheral side area and an outer peripheral side areacontiguous to said principle surfaces, each of the peripheral side areahaving a side end wall and intermediate regions between the side endwall and the principle surfaces, said principle surfaces having smoothsurfaces which are suitable for use in a magnetic recording mediumwherein, at least one of the intermediate regions and the side end wallof each of the peripheral side area is formed by a mirror finishedsurface which have a surface roughness Ra not greater than 1 μm in orderto substantially reduce particles generated from the peripheral sidearea, where Ra is representative of the center-line mean roughness.
 4. Adisk-shaped glass substrate as claimed in claim 1, wherein: the surfaceroughness Ra falls within a range between 0.001 and 0.8 μm.
 5. Adisk-shaped glass substrate as claimed in claim 1, wherein: the surfaceroughness Ra falls within a range between 0.001 and 0.5 μm.
 6. Adisk-shaped glass substrate as claimed in claim 1, wherein: saidprinciple surfaces have the surface roughness Ra which is not greaterthan 2 nm.
 7. A disk-shaped glass substrate as claimed in claim 1,wherein: said glass substrate is at least one selected from the groupconsisting of aluminosilicate glass, soda-lime glass, sodaaluminosilicate glass, aluminoborosilicate glass, borosilicate glass,silicate glass, chain silicate glass, and crystallized glass.
 8. Adisk-shaped glass substrate as claimed in claim 1, wherein: said glasscomprises aluminosilicate glass, the aluminosilicate glass contains, byweight, 62-75% of SiO₂, 5-15% of Al₂O₃, 4-10% of Li₂O, 4-12% of Na₂O,5-15% of ZrO₂ as main components, and a weight ration of Na₂O/ZrO₂ fallswithin the range between 0.5 and 2.0 while a weight ratio of Al₂O₃/ZrO₂falls within the range between 0.4 and 2.5.
 9. A disk-shaped glasssubstrate as claimed in claim 1, wherein: at least the principlesurfaces are chemically reinforced.
 10. A disk-shaped glass substrate asclaimed in claim 1, wherein: said intermediate regions are chamfered sothat rounded surfaces are formed between each of said principle surfacesand each of said intermediate regions, said rounded surfaces having aradius between 0.2 and 10 mm.
 11. A disk-shaped glass substrate for usein a magnetic recording medium, said glass substrate having a pair ofprinciple surfaces opposite to each other and an internal peripheralside area and an outer peripheral side area contiguous to said principlesurfaces, each of the peripheral side area having a side end wall andintermediate regions between the side end wall and the principlesurfaces, said principle surfaces having smooth surfaces which aresuitable for use in a magnetic recording medium, wherein, at least oneof said intermediate regions and said side end wall of at least theouter peripheral side area have a surface having a surface roughnessRmax not greater than 4 μm in order to substantially reduce particlesgenerated from the peripheral side area, where Rmax is a differencebetween a maximum height and a minimum height.
 12. A disk-shaped glasssubstrate for use in a magnetic recording medium, said glass substratehaving a pair of principle surfaces opposite to each other and aninternal peripheral side area and an outer peripheral side areacontiguous to said principle surfaces, each of the peripheral side areahaving a side end wall and intermediate regions between the side endwall and the principle surfaces, said principle surfaces having smoothsurfaces which are suitable for use in a magnetic recording medium,wherein, at least the intermediate regions of at least the outerperipheral side area have a surface having a surface roughness Rmax notgreater than 4 μm in order to substantially reduce particles generatedfrom the peripheral side area, where Rmax is a difference between amaximum height and a minimum height.
 13. A disk-shaped glass substratefor use in a magnetic recording medium, said glass substrate having apair of principle surfaces opposite to each other and an internalperipheral side area and an outer peripheral side area contiguous tosaid principle surfaces, each of the peripheral side area having a sideend wall and intermediate regions between the side end wall and theprinciple surfaces; said principle surfaces having smooth surfaces whichare suitable for use in a magnetic recording medium, wherein, at leastone of the intermediate regions and the side end wall of each of theperipheral side area have a surface having a surface roughness Rmax notgreater than 4 μm in order to substantially reduce particles generatedfrom the peripheral side area, where Rmax is a difference between amaximum height and a minimum height.
 14. A disk-shaped glass substrateas claimed in claim 11, wherein; the surface roughness Rmax falls withina range between 0.01 and 2 μm.
 15. A disk-shaped glass substrate asclaimed in claim 11, wherein: the surface roughness Rmax falls within arange between 0.01 and 1 μm.
 16. A disk-shaped glass substrate asclaimed in claim 11, wherein: said principle surfaces have the surfaceroughness Rmax which is not greater than 2 nm.
 17. A disk-shaped glasssubstrate as claimed in claim 11, wherein: said glass substrate is atleast one selected from the group consisting of aluminosilicate glass,soda-lime glass, soda aluminosilicate glass, aluminoborosilicate glass,borosilicate glass, silicate glass, chain silicate glass, andcrystallized glass.
 18. A disk-shaped glass substrate as claimed inclaim 11, wherein: said glass comprises aluminosilicate glass, thealuminosilicate glass contains, by weight, 62-75% of SiO₂, 5-15% ofAl₂O₃, 4-10% of Li₂O, 4-12% of Na₂O, 5-15% of ZrO₂ as main components,and a weight ration of Na₂O/ZrO₂ falls within the range between 0.5 and2.0 while a weight ratio of Al₂O₃ZrO₂, falls within the range between0.4 and 2.5.
 19. A disk-shaped glass substrate as claimed in claim 11,wherein: at least the principle surfaces are chemically reinforced. 20.A disk-shaped glass substrate as claimed in claim 11, wherein: saidintermediate regions are chamfered so that rounded surfaces are formedbetween each of said principle surfaces and each of said intermediateregions, said rounded surfaces having a radius between 0.2 and 10 mm.21. A disk-shaped glass substrate as claimed in claim 11, wherein: atleast one of the intermediate regions and the side end wall of at leastthe outer peripheral side area is formed by a mirror finished surface.22. A disk-shaped glass substrate for use in a magnetic recordingmedium, said glass substrate having a pair of principle surfacesopposite to each other and an internal peripheral side area and an outerperipheral side area contiguous to said principle surfaces, each of theperipheral side area having a side end wall and intermediate regionsbetween the side end wall and the principle surfaces, said principlesurfaces having smooth surfaces which are suitable for use in a magneticrecording medium, wherein, at least one of said intermediate regions andsaid side end wall of the peripheral side area have a surface having asurface roughness Ra not greater than 1 μm and a surface roughness Rmaxnot greater than 4 μm in order to substantially reduce particlesgenerated from the peripheral side area, where Ra and Rmax arerepresentative of the center-line mean roughness and a differencebetween a maximum height and a minimum height.
 23. A method ofmanufacturing a disk-shaped glass substrate for use in a magneticrecording medium, comprising the steps of: preparing a glass substratehaving a pair of principle surfaces opposite to each other and aninternal peripheral side end surface and an outer peripheral side endsurface contiguous to said principle surfaces; chamfering the outer andinner side end surfaces to make an internal peripheral side area and anouter peripheral side area having a side end wall and intermediateregions between the side end wall and the principle surfaces,respectively; mirror finishing the peripheral side areas by polishing atleast one of the intermediate regions and the side end wall of at leastthe outer peripheral side area in order to substantially reducegeneration of particles from the peripheral side area; lapping thesurfaces of the principle surfaces; and polishing the surfaces of theprinciple surfaces after said lapping step.
 24. A method as claimed inclaim 23, wherein: the step of mirror finishing the peripheral sideareas is carried out after the step of lapping the surfaces of theprinciple surfaces.
 25. A method as claimed in claim 23, furthercomprising the following step: chemically reinforcing at least thesurface of the principle surfaces.
 26. A method as claimed in claim 23,wherein: the step of mirror finishing the peripheral side areas iscarried out by mechanical polishing.
 27. A method as claimed in claim23, wherein: the step of mirror finishing the peripheral side areas iscarried out by polishing by the use of a brush.
 28. A method as claimedin claim 23, wherein: the step of mirror finishing the peripheral sideareas includes chamfering the intermediate regions to provide roundedportions between said principle surface and said intermediate region.29. A method as claimed in claim 23, wherein: said rounded portions havea radius of 0.2-10 mm.